EFFECT OF FLUORESCENCE QUENCHING BY STIMULATED-EMISSION ON THE SPECTRAL PROPERTIES OF A SOLVENT-SENSITIVE FLUOROPHORE

We examined the emission spectra and wavelength-dependent anisotropies of the solvent-sensitive fluorophore 4-(dimethylamino)-4'-cyanostilbe ne (DCS) under condition of fluorescence quenching by stimulated emiss ion. The sample was illuminated with a train of 10 ps pulses at 285 nm , and a train of stimulating pulses at 570 nm which were delayed by a time t(d) relative to the excitation. Stimulated emission of DCS was d emonstrated to occur by observation of gain in the long-wavelength bea m. Illumination on the long-wavelength side of the emission spectrum w ith the long-wavelength time-delayed pulses resulted in a blue shift o f the emission spectrum, and a progressive decrease of the emission an isotropy as the observation wavelength increased toward the stimulatin g wavelength. The spectral shifts and wavelength-dependent anisotropie s of DCS were more pronounced in more viscous solvents where spectral relaxation is incomplete during the excited state lifetime. Light quen ching of DCS in a low-viscosity solvent revealed no spectral shifts or wavelength-dependent anisotropies. Control measurements using acridin e orange, which is relatively insensitive to solvent polarity, did not show any spectral shift or wavelength-dependent anisotropy with light quenching. The data for DCS can be explained by the presence of a tim e-dependent spectral shift and wavelength-selective quenching of the l onger wavelength emission. In this model the relaxed state is formed f ollowing excitation of the unrelaxed state, and the relaxed state is p referentially quenched by long-wavelength illumination, Comparison of the data with model calculations indicates the presence of at least tw o spectral relaxation times. These results demonstrate that light quen ching by stimulated emission acts selectively based on overlap of the stimulating wavelength with the emission spectrum. Observation of the emission spectrum in the presence of time-delayed and power-controlled long-wavelength pulses can be used to study time-dependent excited st ate processes.